Piezoelectric knock sensors are widely used in knock detection of engine blocks or cylinders. The automobile explosion-proof system determines the knocking intensity by detecting the voltage value of the piezoelectric sensor. This article refers to the address: http://
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Therefore, the knocking sensor test system that has the functions of perfect function, stable performance, simple operation and high test precision, which can meet the needs of mass production testing has become the focus of research and development.
Introduction to the automobile knock sensor In order to prevent the damage caused by the knocking phenomenon to the automobile engine, the knock detection of the engine body or the cylinder can be realized by the automobile knock sensor. According to different principles, the knock sensor can be divided into two types: magnetostrictive electronic products and piezoelectric type. The working principle of the piezoelectric knock sensor is: when the engine block vibrates and the vibration is transmitted to the sensor housing. When the outer casing and the weight are moved relative to each other, the piezoelectric element sandwiched between the two is subjected to a change in pressure, thereby generating a voltage. The vehicle explosion-proof system detects the voltage and judges the knock intensity based on the magnitude of the value.
The working principle of the test system The car knock sensor test system described in this paper is mainly used for the final test of the piezoelectric knock sensor. It can detect the internal resistance, insulation resistance, capacitance, sensitivity and linearity of the sensor. Among them, the resistance, capacitance and other indicators can be measured by the corresponding measuring instrument, and the sensitivity and linearity test is difficult to achieve. One way to express the sensitivity of a piezoelectric knock sensor is the ratio of its output voltage to the vibration acceleration at a certain vibration frequency. The system uses the back-to-back comparison method to detect the sensitivity of the knock sensor, that is, using a standard acceleration sensor as a reference, the knock sensor and the standard acceleration sensor are installed back to back, and the output voltages of the two are synchronously measured under the same acceleration, due to the standard acceleration sensor. Sensitivity is known, so the sensitivity of the knock sensor at this frequency can be determined by the ratio of the output voltages of the two sensors.
The linearity detection of the piezoelectric knock sensor mainly changes the vibration frequency of the vibration table by continuously changing the vibration frequency of the vibration table, that is, the vibration acceleration of the vibration table is stabilized within the set range, and the sensitivity of the sensor at each frequency is detected. A continuous curve corresponding to each sensitivity and frequency is outputted, and the curve is tested to meet the linearity requirement of the sensor product. The test flow is shown in FIG. 1 .
In order to meet the needs of accurate measurement of sensitivity, the signal output function of the signal lock-in amplifier is used in the system to provide a sinusoidal excitation signal of a certain frequency. After the filter circuit processing and the power amplifier are amplified, the vibration stage is driven to vibrate according to the set frequency. The software controls the amplitude of the excitation signal voltage through the closed-loop mode, so that the vibration table generates stable acceleration vibration at the frequency, and realizes the synchronous measurement of the two signals of the knock sensor and the standard acceleration sensor by using the phase lock function of the signal lock-in amplifier to ensure the simultaneous measurement. The system linearity testing process is accurate and efficient.
The composition of the test system 1. The hardware composition is to ensure the reliability, high efficiency and scalability of the vehicle knock sensor test system. The PXI system is used to realize the control of the whole system in hardware composition. The hardware resources and system composition are shown in Figure 2. The display mainly includes PXI controller, PXI digital I/O board, vibration table, LCR meter, high resistance meter, signal lock-in amplifier, signal channel switching module and pneumatic fixture system.
Among them, PXI controller is the control core of the whole test system. It can communicate with LCR meter, high resistance meter, signal lock-in amplifier and other instruments through GPIB bus. PXI digital I/O board is mainly used for system feedback and action. Control; the vibrating table provides a standard vibration test environment for the sensitivity and linearity of the knock sensor. The system uses a customized piezoelectric vibrating table with an output frequency range of 3 to 50 kHz and an acceleration of up to 250 g under a load of 1 kg. Vibration; LCR meter, high resistance meter is mainly used for sensor resistance, insulation resistance, capacitance and other parameters detection; signal channel switching module mainly for the resistance capacitance measurement, insulation resistance, sensitivity test task to establish a corresponding signal measurement channel.
Sensitivity and linearity are detected on the production line. The connection between the sensor and the vibrating table is required by tightening the screw. To ensure the consistency of the test conditions, a fixed torque wrench is used to provide a fixed tightening force for the fastening screw. In order to protect the vibrating table, a special pneumatic clamp system is designed in the system to protect the vibrating table during the loading and unloading of the sensor.
2. Software realizes the measurement and control software of the automobile knock sensor test system based on LabVIEW for design and development, with good stability and scalability. The system measurement and control software mainly adopts the parallel processing structure based on the state machine, and combines the event response and the data flow execution mode to make the whole system have high real-time and reliability. The software adopts a modular approach in function implementation. The main components include measurement drive module, vibration table control module, pneumatic clamp control module and system security protection module (see Figure 3).
Among them, the measurement drive module mainly controls the instrument to realize the measurement function; the vibration table control module adopts the closed-loop control algorithm, so that the vibration table can have stable acceleration vibration at different vibration frequencies; the pneumatic fixture control module mainly realizes the vibration table during the test process. The movement control of the fixture can control the movement of the pneumatic system such as the marking mechanism; the external operation drive module monitors the various operational states of the cylinder through the monitoring of the digital I/O port, and uses the three-color warning light to test the test process and test. The results are visually instructed, and the photoelectric sensors on the waste bin can be used to realize the waste counting and quality control; the data analysis and display module can dynamically and intuitively display the detection parameters, test procedures, measurement data and waveforms, and final results. At the same time, the data storage and output module is used to store the test results in an Excel file in real time, and the data can be viewed through the data playback function.
Test system performance and test range The car knock sensor test system developed by our company has perfect functions, stable and reliable performance, simple operation, high test accuracy and fast test speed, which can meet the needs of long-term and large-scale production test on the sensor production line. In view of the various structural characteristics of the piezoelectric knock sensor, the system adopts the modular design of the sensor adapting interface, which is suitable for the detection of a variety of piezoelectric knock sensors. The test range of the system is shown in the table. At present, the test system has been successfully applied to the final test of the Siemens piezoelectric knock sensor production line.